The invention relates to a permanent memory structure, which can be thermo-optically inscribed or written and optically read.
It more particularly applies to data storage means having an optical access. The invention may be described with reference to the non-limitative example of disks known under the generic term optical disks on which data are thermally recorded and permanently stored and are optically read. These disks have an inscribable layer, whose local modification constitutes the recording of the data.
The stored data can be in analog or binary form. Both cases fall within the scope of the invention. The stored data are read by processes based on transmission through the inscribable layer or on the diffraction on said layer of a light beam, followed by conversion into electrical signals.
According to the prior art an optical disk is read by transmission or reflection of a light beam, generally a laser beam through the disk support material which is transparent and covered with an absorbent and reflecting layer. Writing or inscription takes place by thermal ablation of the absorbent layer in which a hole is formed. The reading light beam passes through this hole and supplies an electric signal, whilst its reflection by the layer corresponds to the absence of a signal in the case of reading by transmission. If reading is by reflection, the electrical response is reversed.
The disadvantage of this inscription process by ablation is that the edges of the holes are not planar and clearly defined, so that a background noise occurs during the reading of the optical disk. During inscription the absorbent layer is not volatilized and is instead melted. The molten material is located around the holes in the form of a collar where it is regrouped as a result of capillarity forces in the molten material.
Moreover, it is difficult to duplicate these disks, because it requires a perfect reproduction of the holes in a layer, which itself only has a thickness of a few dozen nanometers. Moreover, it transforms an amplitude contrast into a phase contrast, which in certain cases is disadvantageous for reading purposes.
According to another prior art form the absorbent layer, i.e. the writing layer of the optical disk is only deformed as a result of thermal expansion or deterioration of an underlying polymer layer which expands under the action of the heat of an inscription light beam.
In the case where only the absorbent layer is deformed the reading of the video disk takes place by reflection, either with respect to one fact of the disk by reading the protuberances of the inscription layer, or by the other face of the disk and across the support thereof whilst reading the grooves or slots of the inscription layer.
The disadvantage of said second optical disk structure is that although duplication is possible, it requires two preparation stages with different technologies, one for the underlying polymer layer and the other for the metal layer. Moreover, and as will be shown hereinafter in greater detail, this structure is in certain cases very fragile and difficult to manipulate.